Rotary helical actuators are well known in the art. Basically, these devices apply pressurized fluid to opposite sides of a piston within a hydraulic cylinder. A shaft is rotatably supported within the cylinder. Motion of the piston through selective pressurization of opposite sides of the piston reciprocates the piston within the cylinder. Between the piston and the shaft are helically splined motion conversion means which take the reciprocatory movement of the piston and convert it to the rotary motion of either the shaft or the cylinder, depending on whether the shaft or cylinder is held stationary. Helical splines have the advantage of providing very high torques, and are useful for numerous applications, including articulating rock drill heads or combined pivots and actuators for swing booms on loaders and the like. For applications such as the combined pivot and actuator for swinging the boom of a front end loader, compactness and ability to carry high radial loads are essential. The space available for an actuator at the pivot of a swing boom is very limited. Therefore, the length of the actuator is confined to very short dimensions. This short axial length requirement and very high radial loading have limited the application of rotary actuators for pivots on the boom of a front end loader.
A second problem with actuators which must deliver high torque between the cylinder and the shaft of the actuator is that impacting of the piston at the extreme ends of its travel causes damage to the piston and cylinder. U.S. Pat. No. 4,015,728 discloses hydraulic cushioning valves, but the valves extend into the piston path and will become worn by impact with the piston.